Saturday, August 05, 2023

The Basics of Weather..the terminologies we read everyday:

Rain variability ("vagaries")

One of the most commonly used words to describe the erratic nature of the monsoon is "vagaries", used in newspapers, magazines, books, web portals to insurance plans, and India's budget discussions.

In some years, it rains too much, causing floods in parts of India; in others, it rains too little or not at all, causing droughts. In some years, the rain quantity is sufficient but its timing arbitrary. Sometimes, despite average annual rainfall, the daily distribution or geographic distribution of the rain is substantially skewed.

In the recent past, rainfall variability in short time periods (about a week) were attributed to "Climate Change".

Before we announce as "Climate Change" happening all over, let us understand the Basic:

The difference between weather and climate

The difference between weather and climate is neatly summed up in the saying “the climate is what you expect; the weather is what you get”, frequently attributed to Mark Twain.

Climate is the average weather – for example the mean annual rainfall in Mumbai or the average January temperature in Delhi, or how many cyclones usually happen in the Indian Ocean each year. 

However in any one year, it might be wetter or drier than usual in Mumbai, colder or warmer than usual in Delhi or there may be more or less cyclones, but this won’t necessarily affect the average unless there is a string of years the same.

First let us study what the Basics of weather terms mean...

Rain VS Showers: What is the difference between the terms Rain and Showers.

When you hear or see "showers" on the forecast do you think, there won’t be very much and when you the term "rain" is used do you think, it will be soaking?

The difference between the two is kind of tricky and subtle. There is no doubt that showers are indeed rain. Taking at face value the term rain or showers, has nothing to do with how much precipitation is going to fall. Instead it tells you how it is going to fall. It has to do with the type of cloud they come from.

What are showers?

A shower is a short duration event, that can last a couple of minutes to perhaps 15 minutes or so. But they can sometime last over half an hour. They typically start quickly and end quickly. There can be heavy downpours when dealing with showers.

Showers come from Cumuliform clouds - Cumulus or Cumulonimbus (thunderstorm) the puffy ones that look like they are bubbling up. often separated by blue sky. Showers are pushed around by the wind, so you only experience a particular shower if you are in its path. Since they are hit and miss, your house could be getting wet, while your next-door neighbor could be dry. Cumulus normally result in lower totals while amounts can get quite high from Cumulonimbus.

The customary way of talking about them is isolated showers or scattered/widespread showers. Because they are hit and miss you will never see 100% chance of showers.


What is rain?

Rain is a moderate to long duration event. That can last for a couple of hours to all day. It typically starts gradually then ramps up and ends gradually.

Rain comes from Stratiform clouds- Altostratus and Nimbostratus. These types of clouds are more or less featureless and cover the sky in a grey, widespread sheet, with little to no blue sky to be seen, and of the two, Nimbostratus is thicker and produces heavier rains.

Rain covers a wide area, so most or everyone over a large area are getting wet. Rain can come down lightly or heavily. Because of its long-lasting duration, it can lead to flooding issues.

Now, another Term..Low Pressure:


We read so much about a "Low Pressure" or a "High Pressure" forming...What do we understand by that ..?

We live on a planet that rotates, so this simple wind pattern is distorted to such a degree that the air is twisted to the right of its direction of motion in the Northern Hemisphere and to the left in the Southern Hemisphere.  Today we know this effect as the Coriolis Force and as a direct consequence, great wind spirals are produced which we know as high and low pressure systems.

Why do we generally associate high pressure with fair weather and low pressure with unsettled weather? 

Low Pressure forms When decrease in the air pressure occurs, and the air tends to rise into the higher levels of atmosphere where temperatures are colder.  As the capacity to hold water vapor diminishes, the vapor rapidly condenses and clouds (which are composed of countless billions of tiny water droplets or, at very high altitudes, ice crystals) will develop and ultimately precipitation will fall.   (Of course, we could not forecast zones of high and low pressure without employing some sort of device to measure atmospheric pressure.)

Conversely, High pressure systems are “domes of density” that press down, while low pressure systems are akin to “atmospheric valleys” where the density of the air is less.  Since cool air has less of a capacity to hold water vapor as opposed to warm air, clouds and precipitation are caused by cooling the air. 

So by increasing the air pressure, the temperature rises; underneath those high pressure domes, the air tends to sink (called “subsidence”) into the lower levels of the atmosphere where temperatures are warmer and can hold more water vapor.  Any droplets that might lead to the formation of clouds would tend to evaporate.  The end result tends to be a clearer and drier environment.

In the Northern Hemisphere, the air in low pressure areas spirals counterclockwise and inward — hurricanes, for instance, are Coriolis mechanisms, circulating air counterclockwise.  In contrast, high pressure systems the air spirals clockwise and outward from the center.  In the Southern Hemisphere the direction of the spiraling of the air is reversed.



Usually, the climate is defined as the average weather from the past 30 years

The weather can change from week to week or year to year, whilst the climate remains the same,We live on a planet that rotates, so this simple wind pattern is distorted to such a degree that the air is twisted to the right of its direction of motion in the Northern Hemisphere and to the left in the Southern Hemisphere.  Today we know this effect as the Coriolis Force and as a direct consequence, great wind spirals are produced which we know as high and low pressure systemsIn the Northern Hemisphere, the air in low pressure areas spirals counterclockwise and inward — hurricanes, for instance, are Coriolis mechanisms, circulating air counterclockwise.  In contrast, high pressure systems the air spirals clockwise and outward from the center.  In the Southern Hemisphere the direction of the spiraling of the air is reversed.


2 comments:

sset said...

Once afflicted by droughts, 'desert state' Rajasthan now awash with rain
http://dhunt.in/On5ft

By LokmatTimes English via Dailyhunt

Very interesting article....Rajasthan becoming wetter wetter since last few decades...eastern India becoming drier. Scientists say bipolar shift.

Paresh said...

That was extremely informative and helpful. Thanks Rajesh.

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